There is no particular reason to use a specific TSS in tests that
use task-switching.  In fact, in many cases the tests just want
a separate interrupt stack and could run on 64-bit just as well
if the task-switching is abstracted.

As a first step, remove duplicate protected mode setup from desc.c's
users.  Just leave some spare selectors in cstart.S's GDT before
the CPUs' main TSS.  Then reuse CPU 0's TSS as TSS_MAIN.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>